1. Field of the Invention
The present invention relates to an optical head suitably used for an optical recording medium having a plurality of information recording layers, an optical drive apparatus including the optical head, and a tracking-error-signal generating method.
2. Description of the Related Art
As recording media on and from which digital data is recorded and reproduced, optical discs (including magnetooptical discs), such as CDs (compact discs), MDs (mini-discs), and DVDs (digital versatile discs) are known. An optical disc is a general term of disc-shaped recording media, in which laser light is applied on a thin metal plate protected with plastic, and signals are read on the basis of changes of reflected light.
Optical discs include read-only discs such as CDs, CD-ROMs, and DVD-ROMs, and writable discs on which user data can be written, such as MDs, CD-Rs, CD-RWs, DVD-Rs, DVD-RWs, DVD+RWs, and DVD-RAMs. Data can be recorded on writable discs by, for example, magnetooptical recording, phase change recording, or dye-film change recording. Dye-film change recording is also called write-once recording because data can be recorded thereon only once, and this recording method is suitably used to store data. In contrast, magnetooptical recording and phase change recording allow data rewriting, and have various applications, for example, recording of various content data such as music, pictures, games, and application programs.
Furthermore, high-density optical discs, called Blu-ray Discs, have recently been developed to markedly increase the recording capacity.
High-density discs, such as Blu-ray Discs, include a cover layer of 0.1 mm in thickness. In the discs, phase change marks are recorded and reproduced by using a laser with a wavelength of 405 nm (a so-called blue laser) and an objective lens having a numerical aperture (NA) of 0.85 in combination. When the track pitch is 0.32 μm, the linear density is 0.12 μm/bit, a data block of 64 KB (kilobytes) is a unit of recording and reproduction data, and the format efficiency is approximately 82%, approximately 23.3 GB (gigabytes) of data can be recorded on and reproduced from a disc having a diameter of 12 cm.
When the linear density is set at 0.112 μm/bit without changing the format, 25 GB of data can be recorded and reproduced.
By providing a plurality of recording layers, the capacity can be increased. For example, when two recording layers are provided, the capacity can be increased to double the above-mentioned capacity, that is, to 46.6 GB or 50 GB.